In the following a laser light generating apparatus will be referred to for short as a laser device.
In optically pumped laser devices, a laser beam is generated whenever pumping light absorbed by the laser element excites the latter to a stage that gives rise to a so-called inverted population of atoms. The power of the generated laser beam depends on the energy per unit volume absorbed by the laser element, which in turn depends on the energy, i.e. brightness of the impinging pumping light and the reduction as far as possible of pumping light losses. The brightness of light impinging on the surface of a laser element is limited by the second law of thermodynamics and cannot exceed the brightness of the extraneous light source. Consequently, for achieving good laser action, very bright light sources with suitable imaging and non-imaging concentration optics are usually used.
Depending on design, a laser element may be illuminated at the side, which is referred to in the art as side-pumping (see for example H. Arashi et at., in "A Solar-Pumped cw 18W Nd:YAG Laser", J. Journal Applied Physics, Vol. 23 No. 8, pp. 1051-1053, 1984), or at one of the ends which is referred to in the art as end-pumping (see for example J. Falk, L. Huff and J. D. Taynai, "Solar-Pumped, Mode-Locked Frequency-Doubled Nd: Yag Laser", Conference on laser engineering and appliances IEEE/OSA, May 28-30, 1975).
Side-pumping provides a rather homogenous distribution over the laser element, but due to the usually thin optical density of the laser material, a significant part of the light is not absorbed and the yield is poor.
For end-pumping, the illuminating light is usually concentrated to a spot having a cross-sectional area that at least matches that of the laser element, which provides for absorption of all or most of the pumping light, but the pumping energy in such a case is non homogeneously distributed over the laser element and is limited by the cross-section area of the laser element and by the brightness of the source of pumping light.
The shape of a laser element is selected according to the desired power and quality of the generated laser beam with due regard also to the heat dissipation capability, and in many cases elongated laser elements are preferred with a large length-to-width ratio such as rods, slabs, fibers of solid active material and transparent elongated tubes filled with liquid or gaseous active material. Elongated laser elements are less sensitive to losses in the laser resonator and they create less thermal problems of the kind that are liable to give rise to undesirable side effects. Thus, since excitation occurs throughout the body of the laser element while heat dissipation occurs only at the surface, large thermal gradients are formed which may cause fractures in the element and strongly affect the laser beam quality.
Solid laser elements in the shape of thin elongated bodies such as rods are less susceptible to thermal gradients. However, the use of elongate thin laser elements gives rise to efficiency problems. Thus, when in the case of end-pumping, the cross-sectional area of the illuminating light beam is larger than that of the laser element, there occur significant energy losses due to the fact that not all of the oncoming light impinges on the laser element. Side-pumping of thin rods is also inefficient due to low optical density.
Attempts have been made to improve the lasing efficiency of elongated thin laser elements by concentrating the oncoming extraneous pumping light beam by use of a converging cone. This, however, did not overcome the efficiency problem. Thus, when the cross-sectional area of the exit end of the impinging pumping beam matches the cross-sectional area of the laser element, the input power and with it the output power is limited by the very fact that the cross-sectional area of the laser element is small. If on the other hand, the exit end of the impinging pumping light beam has a cross-sectional area larger than that of the laser element, only a central portion of the oncoming beam impinges on the end portion of the laser element while the askew fringe portions only partly impinge on the side of the laser element and in doing so provide for low power density, even lower than when exclusively side pumping is applied. Depending on the convergence angle of the oncoming beam and the length of the laser element, some of the oncoming pumping light may not impinge at all on the element.
It has also been proposed (G. R. Simpson, "Continuous Sun-Pumped Room Temperature Glass Laser Operation", Applied Optics, Vol. 3 No. 6, pp. 783-784, 1964) to make an optically pumped laser device by placing an elongated laser element within a tubular waveguide having a reflective inner surface. In operation, the cross-sectional area of the incoming activating light beam corresponds to that of the inner hollow space of the tubular waveguide with the consequence that a first portion of the incoming light impinges on the end of the laser element, a second portion which impinges askew on the inner waveguide surface, bounces within the waveguide and across the laser element so as to intersect it at least once thereby producing a side-pumping effect, while a third portion which also impinges askew on the inner waveguide surface, is reflected in such a manner as not to intersect at all the laser element and is thus lost.
It has further been proposed to design a laser device for pumping with concentrated solar radiation by placing a cylindrical rod-shaped laser element within a funnel-shaped frusto, conical envelope having an inner reflective surface and whose narrow end fits snugly over the distal end portion of the laser rod (see C. G. Young, "A Sun-Pumped cw One-Watt Laser", Applied Optics Vol 5, No. 6, pp. 993-997, 1966). The size of the input opening of the funnel around the fore, pumping end of the laser element, is designed to match the cross-sectional area of the incoming pumping beam, e.g. concentrated solar radiation. The pumping efficiency of such a laser is better than of the J. R. Simpson laser device described above, but nevertheless the author mentions that here too a portion of the incident light is rejected by being reflected out of the funnel, while another portion that bounces within the funnel across the laser element, is attenuated in the course of bouncing with the consequence that the output power is low.
It is the object of the present invention to provide an optically pumped laser light generator with an elongated laser element with improved power output.